For semiconductor wafers of silicon, germanium, gallium arsenide (GaAs), etc. for use as the material of IC, LSI or the like, a higher quality of the wafers has been demanded year after year as highly integrated devices are required.
Even when semiconductor wafers are processed by the same processing machine, the quality of each of the wafers may differ to such an extent that badly affects a non-defective rate. Accordingly, for the purpose of improving the quality, it is necessary to accumulate such data that show a correlation between quality and working conditions on the wafer basis.
Conventionally, semiconductor wafers are manufactured on a lot basis (a group of about 20-300 wafers having same characteristics required), and quality information including operation results are managed usually in the form of a daily report in which the quality information is listed by an operator on a lot basis.
However, this method has a problem that important wafer-based quality information such as, from which position of a material ingot, a wafer was cut out, by which thermal treatment machine, the wafer was processed, at which position of a bath of a cleaning equipment, the wafer was cleaned, etc. cannot be managed, thus resulting in lack of the necessary information for quality improvement. Further, since the management of the quality information is manually carried out in this method, this results in reduction of the reliability of the information and in the increased number of necessary management steps.
Furthermore, since the management is carried out on a lot basis, in the case where the next step is such a batch processing step as a cleaning or chemical treatment step, the processing cannot proceed to the next step until one-lot processing is finished, which results in the fact that a wait time is increased and thus a processing time includes a useless waste time, which impedes, in particular, cost reduction in a recent tendency of small number/multiple sorts of items production.
In general, for the purpose of managing objects to be processed on an individual basis, there is employed such a method that a bar code or marker is applied to each of the objects and object management is effected on the basis of the bar code or mark as an identification symbol. When the objects are semiconductor wafers, however, such application of the identification mark to the wafer in the above method is not preferable from the viewpoint of quality.
For example, such bar code becomes one of serious pollution sources at the time of cleaning the wafers. Further, when laser marking is carried out, the mark must be made fairly deep, which might involve fluctuations in the wafer characteristics caused by a high level of laser energy. In addition, pollution substances tend to accumulated in the marks, which might lead to undesirable influences on other wafers. And some products must be subjected to such a step that the marking is never allowed.
In this way, the conventional method has a problem that wafer-basis quality information cannot be managed easily and sufficiently.